A conventional television receiver, such as a receiver according to NTSC broadcast standards adopted in the United States and elsewhere, displays a line interlaced image with a 4.times.3 aspect ratio (the ratio of the width to the height of the displayed image). Recently, significant interest has developed concerning the use of enhanced image display formats including wider image aspect ratios, e.g., 16.times.9 or 5.times.3, and progressively scanned images.
Two prominent types of compatible wide aspect ratio (widescreen) television signal processing systems are the "side panel" system and the "letterbox" system. In the side panel system, left and right image side panels are spliced to a main image panel to produce a widescreen image with a 16.times.9 aspect ratio. One type of NTSC compatible enhanced definition widescreen system is described in U.S. Pat. No. 4,979,020 - Isnardi. The letterbox system is commonly used in Europe for television broadcasting of wide aspect ratio movies. In such case a standard 4.times.3 aspect ratio television receiver displays a wide aspect ratio (e.g., 16.times.9) with black bars appearing along the top and bottom edges of the displayed image. Although the side panel widescreen format avoids the black bars of the letterbox format, signal processing associated with the side panel format is more complex.
Both the side panel system and the letterbox system may display an image in 1:1 progressive scan (line sequential) format rather than 2:1 interlaced scan format to produce what is perceived as a higher resolution display. Often this involves converting a received interlaced image signal to a progressive scan image signal with the assistance of a so-called "helper" signal. For example, in the context of a compatible letterbox system, when the original (source) image information is in progressive scan format, a letterbox encoder subsamples the progressive scan image signal to create an NTSC compatible line interlaced signal for transmission to a standard receiver. The encoder also generates a vertical helper signal prior to subsampling to help convert the compatible letterbox coded interlaced signal back to the original progressive line scan format at a widescreen progressive scan receiver. The helper signal may be transmitted in the bar regions of the compatible letterbox coded signal. At a widescreen receiver, the helper signal recovered from the bar regions allows the widescreen receiver to recover vertical resolution that would otherwise be lost in the conversion cycle from progressive scan to interlaced scan, then back to progressive scan. Techniques for developing and using such helper signals are well known. To prevent the bar regions of a wide aspect ratio letterbox image displayed by a standard aspect ratio receiver from distracting a viewer, i.e., to reduce the visibility of the helper signal, the amplitude of the helper signal may be attenuated and shifted in a black image direction at the encoder to assure that the bar region appears black when displayed, for most if not all images. The helper signal is recovered at a widescreen receiver by performing the inverse of the attenuating and shifting operations performed at the transmitter/encoder.
It is herein recognized that a helper signal can be sufficiently degraded by noise, such as transmission channel noise for example, to adversely affect the quality of a reconstructed image displayed at a receiver. This is particularly likely to occur, for example, when the helper signal and the image signal are subjected to different types of signal processing. In the case of a letterbox system, it is recognized that channel noise is likely to adversely affect a helper signal compressed in the bar regions of a compatible letterbox television signal. A system according to the present invention reduces the effect of noise upon an image reconstructed in response to a helper signal subject to corruption by noise.